1. Field of the Invention
This invention generally relates to a color image forming apparatus, and in particular to a multicolored electrophotographic image forming apparatus in which a plurality of latent image bearing members such as electrophotographic photosensitive members are juxtaposed and these photosensitive members are subjected to the electrophotographic image forming process to obtain developed images of respective colors and the developed images are successively transferred to a transfer material carried and conveyed by conveyor means such as a belt to there-by obtain a colored image. The present invention can be embodied not only in electrophotographic color copying apparatuses but also in various color printers or the like, but herein the present invention will be described in connection with a color electrophotographic copying apparatus. Also, the image bering members, such as those in the present invention, may be endlessly movable image bearing members of any shape such as a belt-like shape or a drum-like shape. For purposes of example, the image bearing members will be described as being of a drum-like shape.
2. Related Background Art
Several types of electrophotographic color copying apparatuses have heretofore been proposed and commercialized. A typical color copying apparatus in which the present invention may be used is illustrated in FIG. 2 of the accompanying drawings.
In the color electrophotographic copying apparatus of FIG. 2, four image forming units Pa, Pb, Pc and Pd are disposed and respectively have exclusive photosensitive drums 1a, 1b, 1c and 1d, around which are disposed exposure means 3a, 3b, 3c, 3d, developing stations 4a, 4b, 4c, 4d, transfer stations 5a, 5b, 5c, 5d and cleaning stations 6a, 6b, 6c, 6d. On the other hand, conveyor means 7 such as an endless belt is disposed through the image forming units Pa, Pb, Pc and Pd and below the photosensitive drums 1a, 1b, 1c and 1d to convey a transfer material 9 fed by paper feed rollers 8 through the transfer stations 5a, 5b, 5c and 5d of the image forming units Pa, Pb, Pc and Pd, respectively.
In such a construction, a latent image of a yellow component color corresponding to the image of an original is first formed on the photosensitive drum 1a by conventional electrophotographic means such as the charging station 2a and exposure means 3a of the first image forming unit Pa, whereafter a visible image is formed at the developing station 3a by a developer having yellow toner, and at the transfer station 4a, the yellow toner image is transferred to the transfer material 9 conveyed thereto by the conveyor means 7.
On the other hand, while the yellow image is being transferred to the transfer material 9, a latent image of magenta component color is formed in the second image forming unit Pb, and subsequently, a toner image by magenta toner is obtained at the developing station 4b, and when the transfer material 9 to which the aforementioned yellow toner image has been transferred in the first image forming unit Pa is conveyed to the transfer staiton 5b of the second image forming unit Pb, the magenta toner image is transferred to a predetermined location on the transfer material 9.
Thereafter, with respect to each of cyan color and black color, image formation is carried out in a similar manner, and when superposition of four color toner images is completed on the transfer material 9, the transfer material 9 is fixed at a fixing station 10 and thus, a multicolored (full-colored) image is obtained on the transfer material 9.
After the termination of the image transfer, the respective photosensitive drums are cleaned by the cleaning means 6a, 6b, 6c and 6d to remove any residual toners therefrom, thus becoming ready for the next cycle of latent image formation.
Such a full-colored image forming apparatus has the merits.
(1) that it is advantageous for high-speed operation because it has respective image forming units for respective colors; and
(2) that it has adaptability for a transfer material such as thick paper or the like because the transfer path can be constructed on a straight line.
However, one of the greatest demerits of such apparatus is that it has many problems in accomplishing the registration between images of respective colors formed by different image forming units.
The deviation between the formed positions of the four colored images transferred onto the transfer material (the misregistration) finally appears as color misregistration or a change in hue. To prevent such misregistration, the following countermeasures are necessary:
(1) To adjust the rotational speeds of the four photosensitive drums 1a, 1b, 1c and 1d to one another; and
(2) To keep the speed of movement of the transfer material constant.
Therefore, in the past, an encoder was directly connected to a drive motor for the photosensitive drums and transfer material conveying means to control the drive motor by PLL control so as to keep constant rotation.
However, if there is an eccentric component in the reduction gear train connected to the drive motor, a speed change will occur to the driven members such as the photosensitive drums. Also, the eccentricity of the photosensitive drums or the eccentric component resulting from the axis deviation between the photosensitive drums and the photosensitive drum driving member will cause a speed change of the surface of the photosensitive drums. Such speed change of the surface of the drums will in turn cause positional deviation of the image writing (for example, image exposure starting) position during the formation of latent images on the photosensitive drums.
The amount of image writing position deviation .DELTA.L is a period function with one full rotation 2.pi. of the photosensitive drums as the period and is therefore given as the following equation by Fourier series, and .DELTA.L varies in the fashion of sine. ##EQU1##
FIG. 3 of the accompanying drawings schematically shows the shape of a photosensitive drum having a radius .gamma.. The center of the drum is O', the center of rotation is O, and accordingly, the amount eccentricity e is e=(OO'). Arrow X indicates the exposure position to the photosensitive drum by exposure means.
Assuming that image writing-out, i.e., exposure, has been started from a state in which, as shown in FIG. 3, the center of the drum has become eccentric from the center of rotation O to the position of an angle .PSI., when the photosensitive drum is further rotated by an angle .theta. from this state, the amount of positional deviation .DELTA.L of the image formed on the drum is such as shown in FIG. 4 of the accompanying drawings.
Irrespective of the magnitude of the angle of deviation .PSI. of the center O' of the photosensitive drum from the center of rotation O, the amount of positional deviation .DELTA.L makes a sine-like change having an amplitude of 2e, but by the change of the angle .PSI. (phase), .DELTA.L is fast or slow relative to the regular position.
From the above-described simulation, the image formed at .theta.=.pi. when a drum having an amount of eccentricity e is put out in the apparatus shown in FIG. 2, for example, when an image is written from the time when the yellow drum is at .PSI.=O and an image is written from the time when the magenta drum is at .PSI.=.pi. is slower by 2e for yellow than the regular position and faster by 2e for magenta than the regular position and thus, the color misregistration between the two is 4e.
That is, in the case of a drum having eccentricity of 0.1 mm, color misregistration of maximum 0.4 mm occurs. Such positional deviation between images by eccentricity holds true not only of the eccentricity of the photosensitive drums, but also of the eccentricity caused by the back-lash during the mounting of the photosensitive drums and the eccentricity of the driving gears.
The positional deviation between images caused by eccentricity can be reduced if the absolute amount of eccentricity is made small, but it requires high accuracy of the drums and high accuracy of the drum driving gears, and this poses a problem in cost. Also, the photosensitive drums and gears are incorporated without any special attention being paid thereto when they are mounted into the apparatus body and therefore, it is difficult to control the eccentricity phase angle .PSI. of the photosensitive drums. Also, even if the first photosensitive drum is carefully incorporated, the phase angle .PSI. differs each time the photosensitive drum is interchanged, and this leads to a problem that color misregistration may occur at a later time.